The present invention is directed to a coating composition, a coated steel product, and a method of making, and in particular, to an aluminum-zinc coating composition employing effective amounts of a particulate compound constituent to enhance tension bend rust stain performance and the appearance of the sheet when painted and reduce spangle facet size.
The coating of steel components with aluminum-based coating alloys, commonly referred to a hot dip coating, is well known in the prior art. One particular type of coating is trademarked as Galvalume(copyright) which is owned by BIEC International, Inc., and is representative of an aluminum-zinc coating alloy.
These materials are advantageous as building materials, particularly wall and roof construction due to their corrosion resistance, durability, heat reflection, and paintability. Typically, these materials are manufactured by passing a steel product such as a sheet or plate through a bath of a melted alloy coating composition comprising aluminum, zinc and silicon. The amount of coating applied to the steel products is controlled by wiping, and then the products are cooled. One characteristic of the coating applied to the steel product is its grain size or spangle facet size.
U.S. Pat. No. 3,343,930 to Borzillo et al., U.S. Pat. No. 5,049,202 to Willis et al. and U.S. Pat. No. 5,789,089 to Maki et al. disclose methods and techniques for the manufacture of steel sheets coated with these aluminum-zinc alloys. The three references are herein incorporated by reference in their entirety.
European Patent Application No. 0 905270 A2 to Komatsu et al. discloses another coating process utilizing zinc, aluminum and magnesium. This application is directed at solving the corrosion problems associated with baths containing magnesium as an alloying element. Further, it is disclosed that the undesirable stripe pattern occurring in magnesium-containing baths does not occur in baths without magnesium.
U.S. Pat. No. 5,571,566 to Cho discloses another method of manufacturing coated steel sheet using an aluminum-zinc-silicon alloy. The object of the Cho patent is to provide a more efficient production method for manufacturing coated steel sheet. Cho meets this object by uniformly minimizing the size of spangles by introducing a large number of spangle particles into the coating which limits subsequent growth of the spangles because these particles interfere with their respective growth resulting in a smaller spangle facet size. The seed effect is achieved by using titanium as part of the molten coating composition.
A similar disclosure with respect to the use of titanium in coating baths to minimize spangle facet size is disclosed in an article entitled xe2x80x9cMinimization of Galvalume Spangle facet size By Titanium Addition To Coating Bathxe2x80x9d, by Cho, presented for the INTERZAC 94 Conference in Canada in 1994. In this article, the author indicates that elements such as titanium, boron, and chromium produce finer spangles in a Galvalume coating, such a disclosure consisted with the disclosure of the Cho patent.
Notwithstanding the improvements suggested by Cho, presently used coated steel product still have disadvantages. One disadvantage is that, when the coated steel product is to be painted, a temper rolling is required to flatten the product in preparation for painting. Another problem is cracking when the product is a sheet and is bent. When this sheet product is bent, the coating can crack, the crack exposing the steel to the environment and premature corrosion. With presently available coated steel sheets, large cracks can form, thereby compromising the corrosion resistance of the sheet product.
In light of the deficiencies in the prior art a need has developed to provide an aluminum-zinc coated steel product with improved bending performance, reduced spangle facet size, and improved painted surface appearance. The present invention solves this need by providing a method of coating a steel product, a coating composition and a coated steel article which, when experiencing surface cracking during bending, is still corrosion resistant and does not require temper rolling when the coated steel product is painted. The coating composition is modified with one or more particulate compound constituents such as titanium boride, aluminum boride and the like.
Accordingly, it is a first object of the present invention to provide an improved hot dip coating composition for steel products.
Another object of the present invention is a method of coating a steel product using a modified aluminum-zinc coating alloy.
Still further objects of the present invention are to provide a coated steel product with enhanced tension bend rust stain performance and painted appearance.
One other object of the present invention is a coated steel article employing a modified coating alloy composition.
Yet another object of the invention is a method of coating and then painting a steel product, whereby the coated steel product does not require temper rolling before painting.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention is an improvement in the art of hot dip coating of steel products using an aluminum-zinc coating alloy. The composition of the aluminum-zinc alloy is modified by adding an effective amount of one or more of a particulate compound constituent selected from the group consisting of boride compounds having one of titanium and aluminum, aluminide compounds containing titanium and iron, and carbide compounds containing titanium, vanadium, tungsten, and iron. Preferably, the constituent is one of TiC, TiB2, AlB2, AlB12, and TiAl13 
The constituent can be prepared in various ways as part of the modification step, e.g., as part of a precursor or master alloy ingot or bath containing principally aluminum, the master alloy then added to an aluminum-zinc bath in the necessary proportions to arrive at a final bath composition suitable for coating and providing the benefits of the invention as a result of the modifier constituent. The constituent can be added to the master alloy as particulate compounds or can be formed in-situ in the master alloy to add to the actual coating bath.
More particularly, the composition of the coating bath can be modified by: (1) directly adding the particles (as a powder) to the coating bath or a pre-melt pot which feeds the coating bath; (2) adding an ingot than contains the required particles; the ingot may be aluminum with particles, zinc with particles, a zinc-aluminum alloy with particles, etc.; the ingot may be added to a main coating pot or a pre-melt pot; (3) adding molten bath containing the required particles, wherein the liquid may be aluminum with particles, zinc with particles, a zinc-aluminum alloy with particles, etc.; (4) in-situ reaction in the main pot or pre-melt pot, for example by the reaction of elemental species, such as titanium and boron in an aluminum feed melt, or the reaction of salts on the feed melt pot to produce particles.
The particle size of the constituent in the coating bath can vary but preferably ranges from about 0.01 and 25 microns. When practicing the invention, a spangle facet size of a coated product can range as low as 0.05 mm and up to 2.0 mm.
The effective amount of the constituent is considered to be that amount which reduces the spangle facet size of the coated product, causes an increase in the number of cracks while maintaining a smaller crack size than conventional aluminum-zinc coated products, and does not require temper rolling when painting. An overall weight percentage range of the constituent, boride, carbide, or aluminide, based on the alloy bath is believed to be between about 0.0005 and 3.5%. When the constituent is a boride, a preferred weight percentage of the constituent as part of the coating bath can range between about 0.001 and 0.5%. When the constituent is a carbide, a preferred weight percentage can range between about 0.0005 and 0.01%.
The invention also provides a coated steel article employing a coating containing the particulate compound constituent as well as the coating composition as applied to the steel product. The product is preferably a steel sheet or plate for construction purposes.